A fuel cell, unlike a conventional battery, is an electricity-generating battery and thus does not need to be replaced or recharged. It oxidizes a fuel such as hydrogen or methanol to generate a chemical energy and then converts the chemical energy into an electrical energy. Since a fuel cell is a electricity-generating device of high efficiency facilitating an energy conversion rate of about 60%, it can remarkably reduce the fuel consumption. Furthermore, it is one of the eco-friendly energy sources without generating pollutants. A fuel cell having such advantages can be applied to various fields, especially to a power supply for a transport such as a vehicle and so on.
A fuel cell may be classified into various types based on the kind of an electrolyte and the operation temperature. Among the various types of a fuel cell, a polymer electrolyte membrane fuel cell (PEMFC) is receiving special attention as a future power supply.
A polymer electrolyte membrane fuel cell comprises an anode, a cathode, and a polymer electrolyte membrane therebetween. Hydrogen or gas including hydrogen is generally used as a fuel to be supplied to the anode. Oxygen or gas including oxygen is generally used as an oxidant to be supplied to the cathode. The fuel is oxidized at the anode to create a proton and an electron. The proton is delivered to the cathode through the electrolyte membrane and the electron is delivered to an external circuit. The proton coming through the electrolyte membrane, an electrode from the external circuit, and oxygen are combined at the cathode to create water.
From the viewpoint of the generating efficiency of a fuel cell or the system efficiency, it is required for the electrolyte membrane to have a good cation conductivity under the conditions of high temperature of 100° C. to 300° C. and low humidity of 50% or less.
However, a sufficient amount of moisture needs to be supplied for the conventional electrolyte membrane formed of a polymer having a sulfonic acid group to perform the proton conducting function well. The conventional electrolyte membrane cannot perform the cation conducting function in a satisfactory manner under the condition where moisture is easily evaporated, i.e., high temperature of 100° C. or higher or low humidity of 50% or less.
To solve the aforementioned problem, hetero ring compounds such as imidazole, pyrazole, and benzimidazole are suggested as a cation conductor which can supersede the water. (Journal of The Electrochemical Society, 2007, 154(4), pp. 290-294).
However, the hetero ring compounds, due to their low molecular weight, are volatile materials which cannot be tightly fixed to an electrolyte membrane, and there has not been suggested any method so far to tightly fix the volatile compounds to an electrolyte membrane.